1. Field of the Invention
The present invention relates to a rubber composition of a tread portion of a pneumatic tire suffering less rubber degradation and alleviated in discoloring of the tire surface in prolonged use.
2. Description of the Background Art
Rubbers such as natural rubber, styrene-butadiene rubber, polybutadiene rubber and nitrile-butadiene rubber have been used widely as the rubber compositions for tread portions and/or sidewall portions of pneumatic tires. Such rubber compositions for tires suffer considerable deformative flexure over a long run and are exposed to high-temperature conditions and ultraviolet radiation, so that they deteriorate with age and become unusable. This phenomenon is usually called “aging”.
Aging of rubber compositions proceeds due to various factors. Among them, external factors include oxygen in the air, oxide, heat, light, ozone, manganese compound, radiation, repetitive deformation during the tire run, and others. Internal factors include kinds of rubber components, conditions on and degrees of vulcanization of rubber, kinds of vulcanizing agents, kinds of vulcanization accelerators, and others. These factors interact with one another to cause the aging of rubber.
Rubber compositions suffer aging primarily due to oxygen and ozone. In the case of aging due to oxygen, hydrocarbon in a rubber molecule generates hydrocarbon radicals and hydrogen radicals in the presence of oxygen under the influence of heat or light. The hydrocarbon radical is reacted with oxygen to generate hydroperoxide, which then is reacted with hydrocarbon in another rubber molecule to further generate hydrocarbon radicals. Such chain reactions cause decomposition of principal chains of the rubber molecules, thereby degrading physical properties thereof.
Diene-based rubbers, such as natural rubber, polyisoprene rubber, polybutadiene rubber and styrene-butadiene rubber, are aged by ozone. Ozone is electrophilically reacted with a double bond of a rubber molecule to cut the double bond portion. This essentially differs from the oxidation described above. The aging due to oxygen or ozone is further promoted by repeated deformative flexure and increased temperature during the tire run.
An antioxidant is conventionally blended in a rubber composition to prevent such aging. With prolonged use of a tire, however, the antioxidant in the rubber composition is consumed as reacted with ozone, heat, ultraviolet radiation and others, and the anti-aging effect gradually decreases. Although the antioxidant of an amount greater than required may be blended in advance, it would discolor the surface of the rubber composition and worsen the appearance.
The rubber composition increasingly hardens with advance of aging. One way to keep the hardness constant is to add oil in the rubber composition. Generally, oil tends to migrate inside the rubber composition, and thus, a method for preventing such migration to maintain uniform hardness over a long period of time has been proposed. For example, in Japanese Patent Laying-Open No. 64-30806, an oil barrier is provided between a tread and a belt of a studless tire. At least 30 weight % of polymer blended in the barrier is formed with halogenated butyl rubber. This method, however, does not alleviate degradation by oxygen or ozone itself. Thus, it cannot suppress degradation of basic properties of the rubber composition, i.e., tensile strength or elongation, grip performance in the tread portion, and others.
In some examples, a re-crosslink inhibitor is blended in a tread portion of a pneumatic tire to restrict changes in physical properties. For example, Japanese Patent Laying-Open No. 2001-30703 discloses a rubber composition including water-soluble fiber and at least one kind of re-crosslink inhibitor selected from dithioic phosphoric acid, thiuram vulcanization accelerator and a compound of 4-methyl benzo thiazole group. Such blending of the re-crosslink inhibitor alone, however, would merely restrict hardening by preventing changes in cross-linked state. It cannot achieve a barrier effect against the influence of external environment.
In other examples, an inorganic filler is blended in the tread portion to improve initial performance. For example, in U.S. Pat. No. 6,035,911, a fluorine-based additive is added to a surface layer of the tread portion of the tire. Although the initial performance may be improved with this method, it cannot prevent performance degradation.